[Bug fortran/54556] New: [4.8 Regression] Marking implicitly pure variables as DECL_PURE_P leads to wrong code

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54556

             Bug #: 54556
           Summary: [4.8 Regression] Marking implicitly pure variables as
                    DECL_PURE_P leads to wrong code
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: burnus@gcc.gnu.org
                CC: Joost.VandeVondele@mat.ethz.ch


See PR 54389 and in particular PR 54389 comment 3.

Joost writes there:
  This revision causes CP2K to produce wrong results at -O1 and above.

Joost, if possible, can you nail down the file in which it occurs? That should
be possible by looking at the .o files whether they change with or without the
patch.


I will try to come up with a scenario where implicit pure gets it wrong.
Combining this with a failing test case is usually the best combination: One
tends to find bugs both ways, but usually different ones.

Besides having a bug in the implicit pure algorithm or in being unable to use
it for this purpose, it might well be that the PURE handling in total has some
bug, e.g. a slightly different semantic between Fortran and the middle end. If
implicit pure is wrong, it will also affect other parts of the code.

In any case, my working hypothesis is a bug in implicit pure. (With partial
compilation, there might be also an issue when the .mod file claims implicit
pure - and then the function gets changed to be no longer (implicitly) pure
without recompilation of its users.)

 * * *

Regarding the patch itself:

http://gcc.gnu.org/viewcvs/trunk/gcc/fortran/trans-decl.c?r1=190757&r2=190756&pathrev=190757

The removal of "|| sym->attr.elemental" fixes a bug. Thus, if you backout the
patch, only remove " || sym->attr.implicit_pure" and do not re-add "elemental".